276 Bulletin, Scripps Institution of Oceanography 
available until the first moorings have been used. The assumption described is 
conservative to the degree that currents do not flow in the same direction. Indeed, 
submerged countereurrents cancel some of the stress in the mooring. The ealeu- 
lation is unconservative to the degree that transient currents may exceed the design 
current in the region. This is accommodated by factors of safety in the design, as 
discussed later. 
It is clear that, with such a current profile, the horizontal drag on the system 
will be stronger when the mixed layer is deeper. In mid-latitudes this effect will 
produce an annual cycle in the dip and the excursion of a subsurface buoy. We 
have had opportunity to apply this rule of thumb in only a few areas in the Pacifie. 
We advise caution in applying it too literally elsewhere, and suggest that all avail- 
able information be studied as a possible basis for a better set of assumptions. 
2) Waves.—The ceaseless motion owing to even small surface waves results in 
chafing, jerking, fouling of slack lines, and ultimate failure. An adequately de- 
signed mooring accommodates this attrition and is protected against premature 
damage or loss. Large waves and especially breaking storm waves (combers) in 
deep water, on the other hand, determine the strength requirements of moorings 
with surface floats because of the large impulsive loadings that the surface com- 
ponents undergo. Deep-moored systems without surface components, such as 
moored sonar targets used in station keeping or submerged recorders that emerge 
or transmit information on command, have much less stringent requirements in 
this respect. 
The water in a nonbreaking deep-sea swell or wave follows an approximately 
circular orbit. Consider the orbiting of a body on the surface of the water as a 
result of wave action. Suppose we assume a wave 20 feet high (H) of 8-second 
period (T). A body would move in a circular orbit 20 feet in diameter with a 
velocity of Hz/T = (20x 3.1) /8=7.8 feet per second. The length of this wave in 
deep water would be approximately 325 feet, and the rate of travel of its crest 
(phase velocity) would be 24 knots (40 feet per second). Figure 3 illustrates the 
WEE Ube | SS 
Float Orbit (20'dia.) 
Wave Height 20° 
Wove Period 8 sec. 
Orbital Velocity 7854 ft. per sec. 
Wave Speed 24 kts (40.8)ft/sec. 
Wave Length=325 ft 
Position of sea surface at successive Mooring Pennant= 300 ft. of buoy- 
“~~ "“two second intervals. ant cable 
See Orbital path of float 
Scole Feet 
SUBMERGED FLOAT 
Pennant 
Fig. 3. Effects of wave motion on mooring with buoyant pennant. 
